Passenger vehicles equipped with spark ignition internal combustion engines operate under widely varying conditions. In order to prevent engine knock over their full range of operation, present ignition systems require that spark timing be significantly retarded from the values yielding best fuel economy and maximum power. In U.S. Patent application Ser. No. 648,313, filed Jan. 12, 1976 by John L. Harned and Donald F. Herrick and assigned to the assignee of this application, a spark timing system is described which includes an engine knock sensor and means to retard spark timing in response to an excess knock signal from the sensor, with the result that the normal spark timing can be set somewhat in advance of the values presently used in order to obtain improved fuel economy and power.
However, although fuel economy and engine power generally improve with advancing spark timing up to the point of maximum acceptable knock, it is found that there is generally an engine spark timing between such maximum knock limited advance and the present normal setting which gives optimum low engine emission characteristics. Thus, an engine with the previously described knock limited spark timing system is normally adjusted with spark timing at the optimum emission values with a closed loop spark retard which only operates with excessive knock. The system, so used, improves fuel economy and power over that of the same engine as presently used; however, it still does not make the most efficient use of the engine. In particular, it is desirable to provide automatic spark advance from the normal settings, when knock is not present, to obtain maximum engine power with wide open throttle, when such power is needed, for example, in passing or accident avoidance situations.